The Long QT Syndrome (LQTS) is a cardiovascular disorder characterized by an abnormality in cardiac repolarization leading to a prolonged QT interval on the ECG. Three of the five genetic loci known to cause the inherited form of the disease code for cardiac potassium channel subunits. One is the Human-ether-a-go-go-related-gene product (HERG) underlying the fast component of the cardiac delayed rectifier IKr. HERG-associated LQT mutations can result either in dysfunctional channels at the plasma membrane, or in incompletely processed channels retained in the endoplasmic reticulum (ER) and then targeted for degradation by ER-resident quality control mechanisms. The forward trafficking of ion channels and other multimeric protein complexes can be regulated through assembly-dependent masking of specific ER retention motifs. One recently identified retention motif found in several membrane protein complexes, is 'RXR' where 'R' is arginine and 'X' can be any amino acid. Our preliminary data indicate that RXR is an important regulator of plasma membrane expression of HERG as well. We found that truncation of 147 amino acids from the distal C-terminus of HERG results in the loss of IKr However, IKr could be restored by deleting an RXR signal immediately upstream of the truncated C-terminus. The central hypothesis of our proposal is that the C-terminus of HERG influences IKr by controlling the rate of HERG trafficking through RXR ER retention signals. Thus, C-terminal LQT truncation mutations reduce the number of HERG channels at the membrane because they expose consensus RXRs recognized by the ER quality control system. To test this hypothesis, we have formulated three Specific Aims: 1. To test whether the exposure an RXR signal in HERG C-terminal LQT mutations leads to an IKr defect due to ER retention. We will determine the effect on HERG trafficking of eight RXR signals in the C-terminus. We will further test whether those RXRs cause defective trafficking of associated downstream LQT mutations. 2. To test if decoy peptides can rescue RXR-dependent ER retention mutants. Following up on our findings that RXR-dependent intracellular retention of HERG can be reversed by treatment with specific peptides, we will develop peptide-based approaches to alleviate ER retention defects of HERG LQT mutations. 3. To identify the mechanism that permits HERG trafficking. We will investigate the role played by the HERG distal C-terminus in masking an upstream RXR, whether shielding of the RXR is accomplished by an intra- or an intermolecular mechanism. This study will explain important aspects of the biogenesis, intracellular processing, and trafficking of an important cardiac K+ channel. We will develop novel therapeutic strategies designed to correct intracellular processing steps of HERG and possibly other plasma membrane proteins that suffer from defects caused by the same mechanism.